A MEASURING DEVICE FOR MEASURING THE INTENSITY AND/OR POLARIZATION OF ELECTROMAGNETIC RADIATION, FOR DETERMINING PHYSICAL PROPERTIES OF A PREPARATION, AND FOR READING INFORMATION FROM A STORAGE MEDIUM

Abstract: A measuring device for measuring the intensity and/or polarization of electromagnetic radiation, more in particular of light. The measuring device is provided with a tunnel tip arranged facing a body at some distance, which tunnel tip at least partly consists of a semiconductor material. The measuring device further comprises a control unit for obtaining information about the intensity and/or polarization of the radiation by means of a tunnel current, generated under the influence of the radiation in the tunnel tip, between the tunnel tip and the body and/or a surface photovoltage of the tunnel tip, generated under the influence of the radiation. The device can be used as near field microscope, polarization selective near field detector. In one special embodiment the body forms an information storage medium.

Description: Title: A measuring device for measuring the intensity and/or polarization of electromagnetic radiation, for determining physical properties of a preparation, and for reading information from a storage medium. This invention relates to a measuring device for measuring the intensity and/or polarization of electromagnetic radiation, more in particular of light. Such measuring devices are known in many variants. In this connection, for instance, CCDs often used in video cameras may be thought of, for one thing, and microscopes, for another. There is, however, a need for a measuring device capable of measuring electromagnetic fields having a very low intensity. Moreover, the resolution of the measuring device should be less than the wavelength of the electromagnetic radiation. This enables details, such as geometric, electric and magnetic properties of a preparation, to be observed on a submicron or even a nanometer scale. A measuring device according to this invention is characterized by providing the measuring device with a tunnel tip arranged facing a body at some distance, which tunnel tip at least partly consists of a semiconductor material and a control unit for obtaining information about the intensity and/or polarization of the radiation by means of a tunnel current, generated under the influence of the radiation in the tunnel tip, between the tunnel tip and the body and/or a surface photovoltage of the tunnel tip, generated under the influence of the radiation. Surprisingly, it has been found that radiation having only a power of 1 m per mm 2 at the place of incidence on the tunnel tip already results in a properly observable tunnel current and/or surface photovoltage. In this connection, for instance, a tunnel current on an order of magnitude of 500 pA and/or a surface photovoltage of 500 mV may be thought of. According to a special embodiment the body, in use, comprises a preparation emitting radiation the intensity and/or polarization of which is determined by the measuring device. The radiation of the preparation detected by the measuring device may then provide relevant physical informa ion about the preparation. More in particular, the above intensity and/or polarization of the radiation is determined for obtaining physical information from the preparation. The manner in which physical. information from a preparation can be obtained from the tunnel current and/or surface photovoltage is known per se, not relevant to the invention and will therefore not be explained in detail. According to a special embodiment the measuring device is further provided with a radiation source with which electromagnetic radiation is emitted in the direction of the tunnel tip for measuring the intensity and/or polarization of the radiation emitted. There can thus be obtained information about the radiation source itself. According to an alternative embodiment of the measuring device the body, in use, comprises a preparation to be investigated, the measuring device further being provided with a radiation source with which electromagnetic radiation is emitted in the direction of the preparation. This radiation emitted will in general undergo an interaction with the preparation which, inter alia, depends on physical properties of the preparation, such as, for instance, the geometry of the preparation and magnetic and/or electric properties of the preparation. The changes experienced by the radiation, such as, for instance, a change of intensity and/or polarization, can be measured very accurately by the measuring device and is subsequently correlated with the above properties of the preparation. According to a possible variant of the invention the preparation is located between the tunnel tip and the radiation source so that the intensity and/or polarization of radiation transmitted by the preparation is measured for determining physical properties of the preparation. According to another variant the radiation source and the tunnel tip are located on the same side of the preparation so that the intensity and/or polarization of radiation at least partly reflected by the preparation is measured for determining physical properties of the preparation. In particular, near-field optical microscopy can be practiced with both variants. In this use, the distance between the tunnel tip and the body will be selected smaller than the wavelength of the radiation. According to a very advanced embodiment of the measuring device according to the invention the tunnel tip is provided with polarization-sensitive means for measuring a predetermined polarization component of the radiation with the tunnel tip. Thus the measuring device can be used as a polarization-selective near-field optical detector. According to a special embodiment the tunnel tip is partly provided for this purpose with a metallic portion, more in particular a metallic coating functioning as a means sensitive to polarization. According to another aspect of the invention the control unit regulates the surface photovoltage within predetermined limits. This surface photovoltage is determined, inter alia, by the distance to be adjusted between tunnel tip and body and the magnitude of the tunnel current. This ensures optimum sensitivity of the tunnel tip. According to another aspect of the invention the control unit generates a bias between the body and the tunnel tip. Thus the tunnel current can be regulated in such a region that in particular small variations in the radiation can be properly measured as variations in the tunnel current and/or surface photovoltage. Accordingly, this enables optimization of the sensitivy of the measuring device. According to a special embodiment of the invention the radiation source emits radiation of a specific wavelength. By selecting the wavelength of the radiation, interesting physical information in relation to a preparation can be obtained on physical grounds. According to another aspect of the invention the radiation source emits polarized radiation. If the polarization of the radiation is changed by a preparation, this can be properly measured with the measuring device according to the invention as described above. It will be clear that on the basis of this measurement information about the magnetic and/or electric properties of a preparation can be obtained. More in particular, this, information is obtained for the bulk of the preparation. According to another aspect of the invention the radiation source therefore emits linearly polarized radiation. According to a special aspect of the invention the radiation source emits circularly polarized radiation. By interaction with the preparation the direction of polarization is often changed, and by measuring this change further information about the preparation can be derived. ith circularly polarized radiation this means that, for instance, elliptically polarized radiation can be obtained, which depends on the electric and/or magnetic properties of the material, in particular of the bulk of the material. Preferably, the radiation source is provided with a modulator for modulating the direction of polarization of the radiation, the control unit comprising a phase-sensitive detector for the phase-sensitive detection, in synchronization with the modulation, of the tunnel current and/or surface photovoltage. This considerably increases the polarization- sensitivity of the measuring device. According to a special use of the invention the body is provided with a preparation of which magnetic and/or electric properties in the bulk of the preparation are determined by means of the tunnel current and/or surface photovoltage. According to another very advanced use of the measuring device according to the invention a preparation is used as a storage medium for information, in particular binary- information, different regions of the preparation being scanned successively with the tunnel tip. Each region can assume an electric and/or magnetic state to be measured. The combination of the electric states of the different regions represent the information stored. In particular, each region only knows two different states so that a bit can be stored per region. According to the invention the lateral resolution, when the measuring device scans a preparation, is less than the wavelength used of the radiation source. Scanning can be effected in a manner known per se. This resolution is determined by the collection volume of the tunnel tip contributing to the tunnel current and/or surface photovoltage generation. By varying the transport properties of the tunnel tip, the above volume can be adapted. This means that tremendous amounts of information can be stored per square centimeter and can then be read. In this connection, for instance, 5 G Byte per square centimeter can be thought of. According to another use of the invention the measuring device is further provided with a control unit-controlled drive for positioning the tunnel tip at the above distance and for scanning the body which, in use, comprises a preparation to be investigated. The preparation to be investigated may also be a preparation the information of which should be read. In this connection, therefore, a preparation to be investigated also includes, in relation to the entire invention, an information carrier to be read. The measuring device can then be characterized in that the preparation to be investigated comprises an information storage medium, position-dependent magnetic and/or electric states of the preparation being measured during scanning, which represent the information stored. Finally, it is observed that the distance between the tunnel tip and the body/preparation will preferably be on an order of magnitude of less than a wavelength of the radiation. Other values, however, are also possible. The tunnel tip can, in principle, be successfully manufactured from many types of semiconductor material, such as, for instance, Si, GaAs, superlattices and heterostuctures. The invention will be explained in detail with reference to the accompanying drawings in which: Figure l shows a first embodiment of a measuring device according to the invention; Figure 2 is a schematic view of an energy band diagram of a tunnel tip arranged facing a body; and Figure 3 shows a second embodiment of a measuring device according to the invention. In Figure l reference numeral 1 designates a measuring device according to the invention. The measuring device is provided with a tunnel tip 2. The tunnel tip 2 is provided with a semiconductor material which in this example is a semiconductor material such as, for instance, n- or p-doped gallium arsenide, but other semiconductor materials such as silicon are possible too. The tunnel tip may also comprise a combination of different materials. Thus, for instance, one or more semiconductors may be applied onto a metal substrate. In this example the tunnel tip 2 is cone-shaped, but other shapes are possible too. A body 4 is placed at a distance d from the tunnel tip 2 and has the property that it can conduct current, and therefore comprises a conductor and/or a semiconductor. Besides, the measuring device is provided with a control unit 6 which is connected at least electrically via line 8 with the tunnel tip 2. The body 4 is also connected with the control unit via line 10. In operation, the measuring device functions as follows. Electromagnetic radiation 12 the intensity and/or polarization of which is to be determined is incident on the semiconductor tip 2. As shown in Figure 2, the semiconductor material of the tunnel tip comprises a valence band VB bending near the surface V of the semiconductor material. The conduction band CB also bends near the surface V of semiconductor material of the tunnel tip. Here at least a part of the semiconductor material is located at the outer surface of the tunnel tip. The band bending region is designated by L. The incident radiation has an energy E=h.f, in which h is the planck constant and f the frequency of the incident radiation 12. When the frequency of the incident radiation is sufficiently high, an electron 14 of the valence band VB will be able to leap to the conduction band CB, an electron hole 16 being created in the valence band. Under the influence of the conduction band, bent downwards in this example, the electron 14 will move to the surface V of the semiconductor material. Thus a current lo is created to which only a limited region of the semiconductor (collection volume) contributes. ' (It will be clear that for semiconductor materials having a conduction band and valence band bent upwards a current can be created in a similar way.) This region is determined, inter alia, by transport parameters of the semiconductor. In this example the surface V of the semiconductor is at a distance d from the surface W of the body 4. In Figure 2 the Fermi level of the body 4 is designated by EFi and the Fermi level of the tunnel tip 2 by EF 2 • Between the tunnel tip 2 and the body 4 there is a potential barrier designated in Figure 2 by EB. In Figure 2 the potential difference between EFi and EF 2 is designated by Vb. In view of quantum-mechanical considerations a part of the electrons 14 will defy the potential barrier EB and flow to the body 4, resulting in a tunnel current Is. The control unit 6 measures the tunnel current Is via lines 8 and 10. The tunnel current Is thus comprises information about the radiation 12. In view of physical laws, which will not be explained here in detail, the tunnel current also comprises information about the polarization of the radiation 12. This involves that according to the invention information about the radiation 12 can be obtained with the measuring device of Figure 1. Here it may be particularly interesting to correlate variations in the tunnel current Is and/or surface photovoltage with variations in the radiation 12. It is also possible with means known per se to measure this tunnel current and/or surface photovoltage and to regulate the same to a predetermined value by varying other parameters the size of which can be correlated with the above intensity and/or polarization of the radiation 12. The parameters to be varied may be, for instance, the distance between tunnel tip and body or the intensity of the radiation source. The variation of the parameters will often be carried out via a feedback circuit to regulate the tunnel current and/or surface photovoltage to the predetermined value. In this example the control unit is provided with a detector 18 connected with the tunnel tip via line 8. Moreover, the control unit comprises a voltage source 20 adjusting the voltage Vb and stabilizing the same in this case via lines 10 and 22. The voltage Vb may be adjusted in dependence on the distance d and the physical properties of the body 4. The detector 18 measures the current is and shows the result thereof on a display 24. The radiation to be detected with the measuring device may originate from different sources, Thus, for instance, it is possible that the radiation is generated by a radiation source not shown, the radiation being detected with the purpose of obtaining information about the radiation of the source and/or information about the source itself. Besides, it is possible that the body 4 comprises a preparation or is completely built up from a preparation which, for any reason whatever, emits radiation. By measuring the intensity and/or polarization of this radiation 12 with the device, physical information about the preparation of the body 4 can be obtained. According to a special embodiment the control unit 6 is further provided with a drive for adjusting the distance d between the tunnel tip and the body 4. Besides, the body 4 can be scanned with the drive 26 by moving the tunnel tip along the surface W of the body. The position of the tunnel tip relative to the body 4 is passed on to the display 24 via line 28 so that there can be obtained a position-dependent picture of the tunnel current Is or other magnitudes connected therewith, in this connection it is observed that the distance d will preferably be much smaller than the wavelength of the radiation. By correlating the tunnel current Is, for instance with the geometry of the body, near-field microscopy can be practiced. The lateral resolution during scanning will then also be less than the wavelength. Since the absolute value of the tunnel current Is is a function of the distance d between the tunnel tip 2 and the surface W of the body, this distance can be regulated and, if necessary, kept constant by supplying the magnitude of the tunnel current via line 30 to the drive 26. Thus, for instance, the magnitude of the tunnel current is kept constant (measuring and regulating) by varying the vertical position of the tunnel tip during scanning so that the tunnel current remains constant. The vertical position of the tunnel tip is then a parameter capable of providing physical information about the body. The distance d will preferably be smaller than the wavelength of the radiation 12. This can be carried out with means known per se. According to another embodiment of the control unit the voltage Vb is regulated to zero. According to a very advanced embodiment of the device the tunnel tip is partly provided at its outer surface with a metallic coating 34, schematically shown in Figure 1 in hatching. Here at least a part of the semiconductor material of the tunnel tip is not covered with the metallic coating. This means that near the metallic coating 34 only electric components of radiation 12 can be measured that are perpendicular to the outer surface of the metallic coating. Thus the measuring device functions as a polarization- selective near-field detector. By carrying out at least three different measurements in which the tunnel tip has different directions, three different electric components are measured so that the polarization, that is to say magnitude and direction of the radiation can be determined. If the radiation relates to light, an optical detector of this type is therefore concerned. Figure 3 shows a special embodiment of the measuring device in which parts corresponding to the parts of Figure l are designated by the same reference numeral. Moreover, this measuring device l is provided with a radiation source 36, the body 4 being placed between the radiation source 36 and the tunnel tip 2. In this example the body comprises a preparation 38 from which the physical information should be obtained. The radiation source emits radiation 40 which penetrates into the tunnel tip 2 via the preparation 38. The radiation 40 leaving the body is designated here by reference numeral 12. The radiation 12, however, will contain physical information about the preparation 38 which can be determined by means of the measuring device by measuring the current Is or the surface photovoltage. Thus, for instance, the intensity of the radiation will depend on the thickness of the preparation and, in case of polarized radiation, on electric and/or magnetic properties of the preparation. If the preparation 38 is scanned by means of the drivα 26, information can be obtained about the thickness of the preparation and the geometry of the surface w of the preparation. If, however, the preparation has polarizing properties affecting the radiation 40, information can be obtained about these polarizing properties by means of the tunnel tip 2 provided with the coating 34. These properties prove to have a direct relation with the electric and magnetic properties of the preparation, both bulk and surface, and adsorbates, if any. More in particular, the radiation source 36 comprises a laser 42, a polarizer 44, and, if required, a modulator 46. The laser 42 emits monochromatic radiation 48 to the polarizer 44. The polarizer 44 emits polarized radiation 40 in the direction of the preparation 38. The radiation can be linearly or circularly polarized in the known manner. If this polarized radiation shines through the preparation 38, the direction of polarization will change in dependence on the magnetic and electric properties of the preparation. The tunnel tip 2 provided with the metallic coating 34 is sensitive to polarization so that these changes of polarization can be measured because they are expressed in the current Is or surface photovoltage determined by the control unit 6. When the tunnel tip 2 is moved with the drive 26 along the surface W of the preparation 38, the magnetic and/or electric properties of the preparation can be mapped and shown on the display 42. In particular, the radiation source 36 is provided with a modulator 46 which modulates the direction of polarization of the polarizer 44. In case of linear polarization this means, for instance, that the vectorial direction of polarization is changed while in case of circular polarization the direction of polarization is changed from levorotatory to dextrorotatory, and vice versa. Moreover, the modulator 46 is connected via line 50 with the detector 18 which now comprises a phase-sensitive detector. Because the current Is or surface photovoltage is phase-sensitively detected, the sensitivity of the measuring device will be improved further. According to an embodiment not shown the radiation source 36 and the tunnel tip 2 of Figure 3 are positioned on the same side of the preparation 38. The radiation source emits radiation in the direction of the end 52 of the tunnel tip 2 and/or the preparation 38. After reflection at the surface of the preparation and, if required, absorption and emission by the preparation this radiation 40 will undergo a change. The radiation 12 detected by the tunnel tip 2 will therefore again comprise physical information about the preparation 38. According to a very special embodiment the preparation 38 forms an information storage medium. The preparation may be located between the radiation source 36 and the tunnel tip 2. It is also possible, however, that tunnel tip 2 and radiation source 36 are located on the same side of the preparation 38. The information is stored in regions of the preparation the electric and/or magnetic state of which can be adjusted. In particular, each region can assume two different states and can therefore comprise a bit of information. Subsequently, by scanning the regions successively with the measuring device, the information stored can be recovered. The lateral resolution may be on an order of magnitude of less than the wavelength of the radiation 40 so that per surface unit large amounts of information can be stored and read. It will be clear that many other uses of the measuring device sensitive to electromagnetic radiation, more in particular light, are conceivable, which have not been and will not be discussed here.

Claims: CLAIMS 1. A measuring device for measuring the intensity and/or polarization of electromagnetic radiation, more in particular of light, characterized in that the measuring device is provided with a tunnel tip arranged facing a body at some distance, said tunnel tip at least partly consisting of a semiconductor material, and a control unit for obtaining information about the intensity and/or polarization of the radiation by means of a tunnel current, generated under the influence of the radiation in the tunnel tip, between the tunnel tip and the body and/or a surface photovoltage of the tunnel tip, generated under the influence of the radiation. 2. A measuring device according to claim 1, characterized in that, in use, only a limited collection volume of the tunnel tip contributes to the generation of the tunnel current and/or surface photovoltage. 3. A measuring device according to claim 1 or 2, characterized in that, in use, the body comprises a preparation emitting radiation the intensity and/or polarization of which is determined by the measuring device. 4. A measuring device according to claim 3, characterized in that said intensity and/or polarization of the radiation are determined for obtaining physical information from the preparation. 5. A measuring device according to any of the preceding claims, characterized in that the tunnel tip is provided with polarization-sensitive means for measuring a predetermined polarization component of the radiation with the tunnel tip. 6. A measuring device according to claim 5, characterized in that the tunnel tip is partly provided with a metallic coating. 7. A measuring device according to claim 5 or 6, characterized in that the control unit regulates the surface ' photovoltage within predetermined limits. 8. A measuring device according to any of the preceding claims, characterized in that the measuring device is further provided with a radiation source with which electromagnetic radiation is emitted in the direction of the tunnel tip for measuring the intensity and/or polarization of the radiation emitted. 9. A measuring device according to any of the preceding claims 1-7, characterized in that, in use, the body comprises a preparation to be investigated, the measuring device further being provided with a radiation source with which electromagnetic radiation is emitted in the direction of the preparation. 10. A measuring device according to claim 9, characterized in that the preparation is located between the tunnel tip and the radiation source so that the intensity and/or polarization of radiation transmitted by the preparation is measured for determining physical properties of the preparation. 11. A measuring device according to claim 9, characterized in that the radiation source and the tunnel tip are located on the same side of the preparation so that the intensity and/or polarization of radiation at least partly reflected by the preparation is measured for determining physical properties of the preparation. 12. A measuring device according to any of the preceding claims, characterized in that the control unit generates a bias between the body and the tunnel tip. 13. A measuring device according to any of the preceding claims 8-12, characterized in that the radiation source emits polarized radiation. 14. A measuring device according to claim 13, characterized in that the radiation source emits linearly polarized radiation. 15. A measuring device according to claim 13, characterized in that the radiation source emits circularly polarized radiation. 16. A measuring device according to claim 14 or 15, characterized in that the radiation source is provided with a modulator for modulating the direction of polarization of the radiation, the control unit comprising a phase-sensitive detector for the phase-sensitive detection, in synchronization with the modulation, of the tunnel current and/or surface photovoltage. 17. A measuring device according to any of the preceding claims 13-16, characterized in that, in use, the body is provided with a preparation of which magnetic and/or electric properties in the bulk of the preparation are determined by means of the tunnel current and/or surface photovoltage. 18. A measuring device according to any of the preceding claims, characterized in that the measuring device is further provided with a control unit-controlled drive for positioning the tunnel tip at some distance and for scanning the body which, in use, comprises a preparation to be investigated. 19. A measuring device according to claim 18, characterized in that the preparation to be investigated comprises an information storage medium, position-dependent magnetic and/or electric states of the preparation being measured during scanning, which represent the information stored. 20. A measuring device according to any of the preceding claims, characterized in that the tunnel current and/or surface photovoltage is regulated to a predetermined value by regulating parameters of the measuring device, said parameters providing information about the intensity and/or polarization of the radiation. 21. A measuring device according to any of the preceding claims, characterized in that the distance between tunnel tip and body is selected smaller than the wavelength of the radiation and that, moreover, the collection volume of the tunnel tip is selected so small that near-field microscopy can be practiced. 22. A measuring device according to claims 5 and 21, characterized in that polarization-sensitive near-field microscopy can be practiced.

Inventors: ABRAHAM DANIEL LEE, JANSEN RONALD, PRINS MENNO WILLEM JOSE, VAN DER WIELEN MAURICE CHRISTI, VAN KEMPEN HERMAN, ABRAHAM, DANIEL, LEE, JANSEN, RONALD, PRINS, MENNO, WILLEM, JOSE, VAN DER WIELEN, MAURICE, CHRISTIENUS, MARTINUS, MA, VAN KEMPEN, HERMAN
Applicants: ABRAHAM DANIEL LEE, JANSEN RONALD, KEMPEN HERMAN VAN, PRINS MENNO WILLEM JOSE, STICHTING KATHOLIEKE UNIV, WIELEN MAURICE CHRISTIENUS MAR, ABRAHAM, DANIEL, LEE, JANSEN, RONALD, PRINS, MENNO, WILLEM, JOSE, STICHTING KATHOLIEKE UNIVERSITEIT NIJMEGEN, VAN DER WIELEN, MAURICE, CHRISTIENUS, MARTINUS, MA, VAN KEMPEN, HERMAN
Application year: 1994
Main classification: G01N27
IPC codes: G01N27, G01R33, G11B9, G11B11, G11B13, G12B21